Spontaneously transformed mouse fibroblasts (Balb/c-3T12-3 cells) were found to reach a lower saturation density when cultured in the presence of retinol or retinoic acid. Retinoids did not influence the number of cell divisions, but only the saturation density, since cultures seeded at different cell density stopped growing when they reached a density of approximately 120,000 cells/sq.cm. Retinoids also caused 3T12-3 cells to become much more adhesive to the culture dish surface and to each other: about 95% or more of the untreated cells were usually lifted from the dish surface by EDTA. Retinoid treatment enhanced the adhesion cells so that only about 5% of them were lifted in this assay. After removal of retinoids from the culture medium, saturation density and adhesion returned to control values within 48 hours.